There are many types of keyboard systems in use in various types of electronic devices, such as mobile devices. Some of these keyboard systems involve the use of a dome sheet structure over a printed circuit board.
The dome sheet typically includes a plurality of domes in a predetermined pattern, which is aligned with a pattern of switch locations on the printed circuitry. When a dome is depressed or collapsed by finger pressure, it causes a corresponding connection to be made in the underlying switch in the printed circuitry. This may be affected by finger pressure directly on the dome or indirectly by finger pressure on a key positioned over the dome.
When the components are clamped together or bonded during manufacturing, air is trapped under each dome. This air must be vented in some manner to enable the dome to be depressed or collapsed in a controlled and smooth manner when pressing a key. Venting is also required to allow the air pressure to equalize in changing atmosphere situations. Venting prevents mushiness and improves the tactile feel of a key when it is depressed.
Some previous keyboards of this type have included a spacer sheet between the dome sheet and the printed circuitry. A series of slots have been die cut in the spacer sheet to allow air to be forced out from under a dome when the dome is depressed or collapsed.
However, on a large multi-keyed keyboard, the presence of numerous slots causes the spacer sheet to be unsteady and difficult to handle and assemble with the other components during manufacturing. Also the use of a separate spacer sheet is disadvantageous in many manufacturing and assembling processes.
Patent publication U.S. Pat. No. 6,491,456 describes a key signal-generating device, which has a thin film circuit board to generate key signals and a plurality of hollow rubber domes set on the thin film circuit board, which domes are compressed by their corresponding keys to activate the thin film circuit board to generate corresponding signals. The rubber domes have a predetermined space that is filled with air. The thin film circuit board has a top surface and a trench on the top surface. One end of the trench extends into the predetermined space so that the predetermined space is in communication with the atmosphere. When a key is pressed, the corresponding rubber dome is squeezed by the key, causing the air in the predetermined space to exit out to the atmosphere through the trench. This kind of a structure causes a dust problem between key dome and circuit board in continuous use, especially when the keyboard is in a mobile device.
Patent publication EP 0 322 515 A describes a keyboard structure (e.g. for a calculator), which includes a base member having a plurality of channels or a chamber, circuitry defining a plurality of switches in a pattern, and optionally a plurality of domes in registry with the switch sites. The dome sheet is arranged over a printed circuit board, which is in turn supported or carried on the relatively rigid base member. When a dome is depressed or deflected downwardly, the air under the dome passes into a channel or a chamber where it is distributed away from the switch site. A vent aperture communicating between the channel or chamber and the atmosphere may also be included. In this structure there is no dust problem, but this kind of a keyboard is rather complex when considering the required manufacturing process.